Environmental Engineering Reference
In-Depth Information
conversion to glucose. Both physical and chemical pretreatments have been used to achieve satisfac-
tory conversion of lignocellulose. Physical pretreatment refers to the reduction of physical size of for-
est biomass feedstock to increase enzyme-accessible surface areas (Lynd 1996; Zhu et al. 2009) and
decrease the crystallinity of cellulose. Chemical pretreatment refers to the process of using chemicals
to remove or modify key chemical components that protect cellulose in biomass, mainly hemicel-
lulose and lignin. Chemical pretreatment often provides a good separation of hemicellulose in the
form of sugars from cellulose for high-value utilization, including liquid fuel through fermentation.
15.6.1 p hySical p rEtrEatmEnt of f orESt B iomaSS —p hySical S izE r Eduction
The issue of physical size reduction has been largely overlooked in the lignocellulosic ethanol
research community. The reason is likely in part that most lignocellulosic ethanol research has
focused on using agricultural biomass that does not need a significant amount of mechanical energy
to achieve satisfactory size reduction. However, size reduction is very energy intensive for forest
biomass. In wood-fiber production, size reduction is in two steps. The first step is coarse size reduc-
tion, reducing wood logs to chips of 10-50 mm in two dimensions and 2-10 mm in the third dimen-
sion. The second step is to further reduce the wood chips to fibers of millimeters in length. Energy
consumption in the first step is much lower than that in the second step.
A simple energy balance calculation using forest biomass demonstrates the importance of size
reduction for biomass refining. Assume that ethanol yield from wood is about 300 L/tonne of oven-
dried wood with current technology. Higher heating value of ethanol is about 24 MJ/L, which gives
total wood ethanol energy of 7.2 MJ/kg wood. Typical energy consumption to produce wood chips
is about 50 Wh/kg; energy consumption in the second step through disk milling can be anywhere
from 150 to 700 Wh/kg (Schell and Harwood 1994), depending on the fiberization process and the
degree of milling. With these assumptions, total size-reduction cost is 200-600 Wh/kg, which is
equivalent to 0.72-2.16 MJ/kg, or 10% to 30% of the wood ethanol energy available.
Three factors affect energy consumption during size reduction: the degree of size reduction,
the fiberization mechanism, and chemical or biological pretreatment before size reduction. All of
these factors also affect enzymatic cellulose saccharification. Most of the existing literature on size
reduction relates to pellet, fiber, and wood flour production. Few studies on biomass size reduction
have taken an integrated approach to examining energy consumption, enzyme-accessible substrate
surface, and chemical pretreatment efficiency in terms of enzymatic cellulose conversion. Most
reported work on size reduction has not involved cellulose conversion (Schell and Harwood 1994;
Cadoche and Lopez 1989; Mani et al. 2004) and has addressed only energy consumption and sub-
strate size. On the other hand, reports on enzymatic hydrolysis using size-reduced substrates did not
provide information about energy consumed to produce the substrate and/or a careful and complete
characterization of substrate size (Allen et al. 2001; Zhu et al. 2005; Nguyen et al. 2000). At most,
substrates were characterized by sieving or screen methods (Sangseethong et al. 1998; Chundawat
et al. 2007; Dasari and Berson 2007; Hoque et al. 2007). Consequently, there is a knowledge gap
linking energy consumption, substrate surface, and pretreatment efficiency.
15.6.1.1 degree of size reduction and substrate-specific surface
To address the degree of size reduction, proper characterization of forest biomass substrate is neces-
sary. The geometric mean diameter of the substrate particles measured by traditional sieving and
screen methods has been almost exclusively used for biomass substrate size characterization (Mani
et al. 2004). This size measure is significantly affected by biomass substrate morphology, such as par-
ticle aspect ratio (Zhu et al. 2009). Most size-reduction processes produce fibrous substrate with a wide
range of particle (fiber) aspect ratio of 5:100. As a result, existing data on substrate size characterization
have limited value. Enzyme-accessible surface area is of most interest for saccharification. Holtzapple
et al. (1989) calculated specific surface area based on a spherical particle assumption to correlate
energy consumption for comparing the efficiencies of several size-reduction processes. The spherical
Search WWH ::




Custom Search